The present invention relates to an objective lens employed in an optical pick-up to be employed in an optical disc device capable of reading/writing from/to two or more types of optical discs having different cover layer thicknesses and data densities.
There exist many types of optical discs according to various standards having different thicknesses of the cover layer (transparent substrate covering the recording surface) and different data densities. For instance, the cover layer thickness of CD (Compact Disc) and CD-R (Compact Disc Recordable) having relatively low data density is 1.2 mm, while that of DVD (Digital Versatile Disc) having relatively high data density is 0.6 mm (xc2xd of that of CD/CD-R).
For the reading/writing of DVDs having high data density, a laser beam having a relatively short wavelength (635-660 nm) is necessary in order to realize a small beam spot diameter. On the other hand, a laser beam having a relatively long wavelength (approximately 780 nm) is necessary for the reading/writing of CD-Rs due to their reflection characteristics.
Since these optical discs are often used in the same optical device, optical disc devices (CD players, DVD recorders, etc.) are desired to support all the optical disc standards. In order to use both DVDs and CD-Rs, the optical disc device is required to have at least two laser diodes for emitting laser beams of wavelengths suitable for the characteristics of the optical discs. Meanwhile, for downsizing the optical disc device, the optical system of its optical pick-up has to be as compact as possible. Therefore, it is preferable that optical elements of the optical pick-up (objective lens, etc.) be shared between two wavelengths and a module containing two laser diodes in one package be employed as a light source unit.
However, if one objective lens is used for each of two types of optical discs which have different cover layer thicknesses as above and spherical aberration is corrected for both discs, it becomes impossible to correct coma for both discs. Therefore, wavefront aberration has its minimum value when the laser beam""s incident angle to the objective lens is 0xc2x0 and increases as the incident angle deviates from 0xc2x0 for both optical discs. Meanwhile, since DVDs with high data density require a large NA (Numerical Aperture), astigmatism changes greatly as the incident angle changes, by which allowable range of the incident angle (in which required performance can be attained) for DVDs becomes narrower than that for CDs. Therefore, the allowable incident angle range in which the wavefront aberration for DVD can be kept within an allowable limit is included in an allowable incident angle range for CD, overlapping with the center of the range for CD.
When a module containing two laser diodes in one package is employed for the light source unit as mentioned above, light emitting points of the two laser diodes are aligned in a direction perpendicular to the optical axis of the objective lens. Therefore, there exists a certain difference of incident angle (to the objective lens) between the two laser beams having different wavelengths, and it is impossible to set both the incident angles (of the two laser beams having different wavelengths) to 0xc2x0 in order to minimize the wavefront aberrations. For the above reasons, the incident angles of the two laser beams have to be properly set in a balanced manner so that the wavefront aberrations for the two optical discs will be within the allowable limit. However, since the allowable incident angle range (in which the wavefront aberration can be kept within the allowable limit) stretches around 0xc2x0 for each laser beam, in order to maintain a preset incident angle difference between the two laser beams, the incident angles to DVD and CD will generally have opposite signs (+/xe2x88x92) and each incident angle is necessitated to be set close to the edge of each allowable incident angle range. Therefore, due to the narrow range of settable incident angle for each laser beam, precise setting is required for the assembly of the elements of the optical pick-up, taking time and trouble. Further, even if the assembly is completed perfectly according to design values, the whole optical system has to be maintained with high accuracy since the wavefront aberration is already close to the allowable limit.
The present invention has been made in consideration of the above problems. According to the present invention, there is provided an optical disc device for an optical pick-up of an optical disc drive by which the aberrations can be reduced and signal reading/writing from/to multiple types of optical discs of different standards can be realized even in the case where a module containing two laser diodes in one package is employed for the light source unit and the optical elements (objective lens etc.) are shared between two wavelengths.
In accordance with a first aspect of the present invention, there is provided an objective lens of an optical pick-up which converges a shorter wavelength laser beam on a first optical disc having a thin cover layer and high data density and converges a longer wavelength laser beam on a second optical disc having a thick cover layer and low data density. The objective lens is formed to provide axial astigmatism corresponding to wavefront aberration of 0.01xcex [rms] or more when the shorter wavelength laser beam is converged on the first optical disc, and to provide abaxial third order coma so that its sign when the shorter wavelength laser beam is converged on the first optical disc will be opposite to its sign when the longer wavelength laser beam is converged on the second optical disc in a virtual state in which the optical axis of the objective lens coincides with the normal of the optical disc.
Since the objective lens is configured to provide the axial astigmatism (astigmatism on the optical axis), an allowable incident angle range (allowable range of the incident angle of a laser beam to the optical disc in which wavefront aberration can be kept within an allowable limit) can be widened for each optical disc even though the minimum value of the wavefront aberration increases. Further, since the objective lens is configured to provide comas of opposite signs for the first optical disc (for which the shorter wavelength laser beam is used) and the second optical disc (for which the longer wavelength laser beam is used), both the wavefront aberrations for the first and second optical discs can be corrected at different incident angles to the first and second optical discs by tilting the optical axis of the objective lens relative to the normal of the optical disc. In other words, two incident angles (to the first and second optical discs) capable of minimizing the wavefront aberrations for the first and second optical discs shift in opposite directions relative to 0xc2x0, and the allowable incident angle ranges (in which the wavefront aberrations can be kept within an allowable limit) for the first and second optical discs also shift in opposite directions relative to 0xc2x0.
Therefore, the allowable incident angle range for the first optical disc and that for the second optical disc can be set in different angle ranges (even if part of them overlapped with each other), differently from the conventional cases where the allowable incident angle range for the first optical disc is totally included in that for the second optical disc. By this, even if a laser module having two light emitting points is employed (that is, even if two laser beams emitted from two light emitting points and incident on the objective lens had a certain incident angle difference), each incident angle to each optical disc can be set in a wide range, by which aberrations can be reduced for both the first and second optical discs and an excellent beam spot can be formed on both optical discs, without the need of precise assembly and setting which used to be necessary for conventional optical pick-ups.
Preferably, the objective lens is formed as an infinite conjugate type lens which focuses the shorter/longer wavelength laser beam incident on the objective lens as parallel light on the first/second optical disc.
When the objective lens is formed by a single lens, it is preferable that one surface of the objective lens be provided with a diffraction lens structure having annular zones which is formed so that spherical aberration provided by the diffraction lens structure will have wavelength dependency such that the spherical aberration changes in an undercorrected direction as the wavelength gets longer.
In accordance with a second aspect of the present invention, there is provided an optical pick-up including a first laser diode which emits a shorter wavelength laser beam, a second laser diode which emits a longer wavelength laser beam, whose light emitting point is placed in the vicinity of that of the first laser diode, and the aforementioned objective lens which converges the shorter wavelength laser beam emitted by the first laser diode on the first optical disc and converges the longer wavelength laser beam emitted by the second laser diode on the second optical disc. In the optical pick-up, light emitting points of the first and second laser diodes are arranged so that focal points formed by the laser beams emitted by the first and second laser diodes will be aligned parallel to one of two focal lines that is formed nearer to the objective lens when parallel light is incident on the objective lens in the direction of the optical axis of the optical system of the optical pick-up, and the optical axis of the objective lens is tilted relative to the normal of the optical disc in a plane including the focal line nearer to the objective lens.
Since the objective lens is configured to have the aforementioned coma and the optical axis of the objective lens is tilted relative to the normal of the optical disc, the comas can be corrected both for the first and second optical discs. The tilt of the optical axis of the objective lens relative to the normal of the optical disc can be realized by two methods: (A) Tilting the optical axis of the whole optical system from the first and second laser diodes to the objective lens relative to the normal of the optical disc; and (B) Tilting the optical axis of the objective lens relative to the normal of the optical disc coinciding with the optical axis of the optical system excluding the objective lens.
In the case of (A), effect caused by the difference of cover layer thickness between the first and second optical discs is canceled out by effect caused by the difference of necessary NA (due to the difference of data density) between the first and second optical discs. For example, when the first optical disc is a DVD and the second optical disc is a CD, the above effects are canceled out almost perfectly, and the change of the coma by the tilting becomes almost equal for the first and second optical discs.
In the case of (B), the change of the coma for the first optical disc by the tilting becomes larger than that for the second optical disc due to the effect caused by the difference of necessary NA.
Since incident angles capable of minimizing the astigmatism are almost equal for the first and second optical discs, it is desirable that incident angles capable of minimizing the coma be set equal for the first and second optical discs; however, the change of the coma by the tilting differs between the cases (A) and (B). Therefore, it is preferable that the amount of coma of the objective lens be changed depending on the setting.
In the case of (A), it is preferable that the abaxial coma for the first optical disc be xe2x88x921 times that for the second optical disc, and the following relationship is desired to be satisfied in a virtual state in which the optical axis of the objective lens coincides with the normal of the optical disc:
xe2x88x922.0 less than SC1/SC2 less than xe2x88x920.5xe2x80x83xe2x80x83(1)
where SC1 denotes offense against the sine condition SC at the outer radius of an effective area when the shorter wavelength laser beam is converged on the first optical disc, and SC2 denotes the offense against the sine condition SC at the outer radius of a common area when the longer wavelength laser beam is converged on the second optical disc.
In the case of (B), it is preferable that the abaxial coma for the first optical disc be xe2x88x922 times that for the second optical disc, and the following relationship is desired to be satisfied:
xe2x88x924.0 less than SC1/SC2 less than xe2x88x921.0xe2x80x83xe2x80x83(2)
The offense against the sine condition SC is defined as:
SC=nH1/(nxe2x80x2 sin Uxe2x80x2)xe2x88x92f(1xe2x88x92m)
where n denotes the refractive index on the incident side, nxe2x80x2 denotes the refractive index on the emerging side, Uxe2x80x2 denotes the angle between the emerging beam and the optical axis, m denotes the paraxial magnification, H1 denotes the ray height on the principal plane, and f denotes the focal length. The common area is defined as an area of the objective lens through which light corresponding an NA necessary and sufficient for the second optical disc passes.
The aforementioned axial astigmatism can be given to the objective lens by forming a surface of the objective lens to have different power depending on the direction, such as a toric surface. When the objective lens is manufactured by injection molding, the astigmatism can be realized either by using a mold with a surface having different power depending on the direction or by properly adjusting the molding conditions.
In accordance with a third aspect of the present invention, there is provided an optical disc device which is provided with a spindle motor which spins an optical disc, the aforementioned optical pick-up, and a driving mechanism which slides at least part of the optical pick-up in the radial direction of the optical disc. In the optical disc device, the position of the optical pick-up relative to the optical disc is set so that the aforementioned focal line nearer to the objective lens will be parallel to tracks of the optical disc when seen in the direction of the optical axis of the objective lens.